Long-term effects of NPK fertilizers and organic manures on carbon stabilization and management index under rice-wheat cropping system

The potential of storing carbon (C) in soil and offsetting the atmospheric CO2 depends on the management practices. This study investigated the long-term (15 years) effects of using nitrogen: phosphorous: potassium (NPK) fertilizers with and without organic manures namely farmyard manure (FYM), green manure (GM), and straw incorporation (SI) on soil C stabilization, and C management index (CMI) in intensive rice-wheat system in Typic Ustochrepts soil of a semiarid region in Punjab, India. The long-term effects of different fertilizer treatments as control, NPK, NPK + GM, NPK + SI, and NPK + FYM on soil organic carbon (SOC), labile fractions of SOC (LFSOC), C stabilization, C sequestration, and C management index (CMI) were determined. Prolonged application of NPK fertilizers alone or in combination with organic manures significantly decreased soil bulk density (from 7.3 to 16.6%) and increased total soil porosity of (from 11.7 to 26.2%) as compared to the control in surface soil (0-15 cm). Fertilizations had positive and variable effects on different LFSOC, namely water soluble C (WSC), labile carbon (LC), particulate organic C (POC), hot water soluble carbon (HWSC), and total organic carbon (TOC). Long-term intensive cultivation and fertilization significantly increased the SOC in surface soil (015 cm) up to 5.16 g kg (-1) as compared with its antecedent value of 2.42 g kg (-1) in 1999. The sensitivity index indicated that among LFSOC, LC (51-85%), WSC (67-131%), POC (64-159%), and HWSC (38-131%) were more sensitive compared with SOC (29-59%). NPK + SI (5.06 Mg C ha(-1)) or NPK + FYM (5.31 Mg C ha(-1)) along with NPK sequestered significantly higher C than the application of NPK (3.57 Mg C ha(-1)) or NPK + GM (4.10 Mg C ha(-1)). The fraction of C in stable fractions (C-frac3 + C-fract4) increased from 7 to 29% with application of NPK fertilizers and organic manures compared with that of the control. The CMI indicated that the use of NPK + SI or NPK + FYM was statistically superior to the use of NPK + GM for improving the SOC status. The study concluded that a relatively high sensitivity index of LFSOC may be used as an early indicator for determining the potential of different management practices for improving SOC on short-term basis. The results indicated that the stability of SOC varies with the nature of the added organic manures, suggesting that the selection of organic manure is important for long-term C sequestration. (C) 2016 Elsevier B.V. All rights reserved.